The present invention relates in particular to the field of semiconductor industry and in particular to the field of “Assembling and Packaging (A+P)”.
It is emphasized, however, that the present invention can also be applied to other fields where it is desired to realize electrical bond connections on copper surfaces.
Therefore, if the present invention is described in the context of the semiconductor industry the invention should not be understood as to be limited to that field. The same problems occur in other fields and the invention can equally be applied therein.
It is common in the semiconductor industry to connect two or more parts, by bonding or by the so-called process of “wire bonding.” One of the parts is customarily a wire and/or comprised of aluminum. The “wire bonding” of parts to form aluminum oxide layers at standard atmosphere is well known in the art. To establish a high quality intermetallic connection between metals, such as between aluminum and gold, where such connection has the lowest possible electrical resistance and is stable and electrically and mechanically reliable, it is necessary to create the connection while heating the parts to at least 80° C., or preferably higher.
However, it is well known that aluminum is not a preferred metal for conducting electricity.
Copper provides a substantially better electrical conductivity and the possibility of smaller dimensioning of current-conducting parts, such as metallic conductors on chips and wire contacts. A great need exists, in particular in the semiconductor industry, to use electrical contact junctions bonded with current-conducting copper parts, as disclosed in Terrence Thompson, “Copper IC Interconnect Update”, HDI, Vol. 2, No. 5, May 1999, p. 42.
However, problems present with the prior art metal bonding processes for copper-gold wire bond systems exist, as disclosed in George G. Harmann, “Wire Bonding in Microelectronics”, McGraw-Hill, 1997, pp. 138-140.
This publication explains (on p. 171) why the bonding of aluminum is relatively free of problems, specifically because a hard brittle oxide layer is formed on it which is forced open through the bonding process. In comparison, softer oxides such as copper and nickel oxide would reduce the capacity for bonding.
Table I-3 “Reversing the Bonded Metallurgical Interface” (p. 128) of the Harmann publication also reveals in principle that hard oxides on soft metals facilitate bonding. This is consistent with the disclosures on pp. 197, 198, according to which it is established that during the bonding, brittle films are forced open and flushed into so-called “disposal zones”. This permits establishing satisfactory ultrasonic and thermionic bond connections through relatively thick layers. The bondability through 2.5 nm CVD deposited oxides is described as being unchanged compared to the bondability of pure contact pads.
In summary, with respect to the semiconductor industry, the bonding of aluminum to copper in the case of conductors permits a further miniaturization of the chips. The fabrication process of chips with copper conductors is well known. Difficulties occur if bond connections of copper contacts of the chips are to be realized to the “chip carrier” (wire bonding, flip-chip technique). In contrast to the aluminum oxide forming on aluminum, which is a hard, thin, oxygen-blocking layer which protects the subjacent metal against further oxidation or contamination at the conventional temperatures during the wire bonding, copper oxide is soft and permits neither the sudden breakthrough during bonding, as does the aluminum oxide, whereby a flux-free welding or soldering connection can be established. Nor does copper oxide form an oxygen diffusion barrier at bonding temperatures.